Hi,
In animals it's best NOT to give something for the pain if you want them to remain inactive. In other words, as Ang said, animals can not be told to stay off your feet for a few hours or couple of days and understand that need to remain inactive to reduce the swelling. Use pain killiers ONLY when you need them to be active, such as need to move those joints (arthritis, etc.) I know it's difficult, but within 48 hrs the swelling will be gone and he'll be bouncing around as normal. The IV produces 2 effects, localized swelling from the amount of fluid in the tissues in the appendage and some vessel dialation where the IV was inserted, these are producing the pain/discomfort and will subside soon. Just allow his natural response to keep him quiet.
Deena